Move preloading behavior to a plugin

Following ideas from #1234:

I’m also wondering if part of the solution here is to convert the preloading features into a plugin and move them out of jspsych.js. This might make it easier to parameterize, and also allows for easier customization by researchers. It would also provide a built-in solution to perform partial caching of materials throughout the experiment. We’ve seen that feature needed in many cases, e.g. #724.

The downside is that automatic preloading wouldn’t be as easy to support. But we could still populate a list of media to preload at the start of the experiment and allow the plugin to access that list. So the experience might be:

var preload = {
	type: 'preload',
	auto_preload: true,
	images: [...],
	audio: [...],
	video: [...],
	show_progress_bar: true,
	loading_message: '<div>whatever html here.</div>',
	show_detailed_errors: true,
	max_load_time: 60000
}

[becky-gilbert]

Yep the code above is from my test. The tests for timelines in objects with conditional and loop functions are working. I can push my changes to the branch tomorrow if that helps.

[jodeleeuw]

I like the success variable in the data. Maybe also a list of failed_images, failed_audio, and failed_video that gives the path to each failed load resource?

Good catch on the preloadAudio! I think this is a good moment to make things consistent.